A theoretical treatment is formulated for transient, or broadband, parametric acoustic sources, i.e., parametric sources using an arbitrarily modulated primary carrier signal. This treatment makes use of previously calculated characteristics for steady-state, or narrow-band, parametric sources. The theory does not include saturation effects. A fortran computer program, using Fast Fourier Transforms, generates spectra and waveforms for specified modulation envelopes. Experimental results, in good agreement with the theory, are presented for a 30-μs pulse applied to a 4-mm active diameter, 2-MHz quartz-crystal underwater projector.